A lead frame is widely used in the assembly stage of the semiconductor manufacturing process, and the lead frame provides external connections between an integrated circuit and a conductive pattern formed on a circuit board.
A punching technique or lithographic techniques are applied to the formation of the lead frame. A blanking die unit is prepared for the punching process, and the blanking die unit usually consists of a punch and a die. The punch is movable into and out of the die, and a lead frame is partially cut out of a thin metal plate on the die.
On the other hand, the lithographic process starts with coating a thin metal film with a photoresist solution followed by an appropriate baking stage. The photoresist film thus provided on the thin metal film is exposed to an ultra-violet radiation through a mask, and the pattern formed in the mask is transferred to the photoresist film. When the photoresist film is partially removed through the development, the thin metal film is in part covered with the photoresist film, but is partially exposed. The thin metal film thus partially exposed is eroded in an etchant, and, consequently, the thin metal film is shaped into a lead frame.
However, a problem is encountered in the prior art with lead frames formed through the punching process in that the production cost is increased due to the preparation of the blanking die unit. The blanking die unit is usually made of a cemented carbide, and the ceramic carbide per se is very expensive and, for this reason, increases the production cost of the lead frame. Moreover, the blanking die unit needs a skilled worker, and plenty of time and labor is required to complete the precision blanking die unit. This further gives rise to increase the production cost of the blanking die unit and, accordingly, the production cost of the lead frame. Residual stress tends to take place in the blanking die unit due to the punching operation, and the blanking die unit suffers from a relatively short service time. This still further increases the production cost of the lead frame.
On the other hand, the lead frame formed through the lithographic process encounters a problem in a low throughput. This is because of the fact that an exposed part of the thin metal film should be perfectly etched away so as to form a predetermined slit pattern. If the etching is carried out from both sides of the thin metal film, the time period may be decreased by half. However, photoresist films are required on both sides of the thin metal film, and the formation of the dual photoresist films makes the process complicated and consumes plenty of time.
Moreover, a lot of inferior products tend to take place due to contamination, and the lithographic process suffers from a low production yield. Namely, the photoresist solution are liable to contain undesirable contaminators, and dust and particles fallen onto the photoresist film in the exposure stage. These contaminators, the dust and the particles are causative of producing pin holes in the lead frame during the etching stage, or, of an incomplete etching operation. This gives rise to a decrease in the production yield of the lead frame. If the working environment is improved, the undesirable contamination will be eliminated from the photoresist film. However, such a solution is extremely expensive, and gives rise to an increase in the production cost of the lead frame.